Some folks, like my friend the Druid, can't stand either the book or the HBO show but I'm frankly addicted to it. In many ways its just a jumped up boddice ripper, but I find it a well told tale with some incredibly gripping scenes. If you haven't seen it, I would definitely say get the DVD of season 1.
So what is it about? Quite a bit really. The short form is it is about royal families fighting and scheming to control the island continent of Westeros. What no one realizes is that there are bigger threats coming.
What else do I need to know? Well, magic is powerful but very rare in this world. There are dragons, and there are things called White Walkers. There's a wall seven hundred feet high made of ice that goes across the norther border to keep the White Walkers out. There is a class of knights that guard the wall that sort of act like Westeros version of the French Foreign Legion. Oh and the seasons are variable and can last for years. Like ten years of winter.
Here is a song that was used in some of the recent trailers. I love it.